The present invention relates generally to scaffold bracket assemblies and more particularly, to the jaws of locks for the bracket assemblies of adjustable height scaffolds which mount on aluminum or fiberglass uprights.
Adjustable height scaffolds are well known in the art and typically comprise four main elements: an upright, a bracket assembly for supporting a work platform on the upright, a jack or block and tackle for raising and lowering the bracket assembly on the upright, and an upright cage for holding the upper end of the upright in place.
Two types of adjustable height scaffolds which are designed for uprights constructed of wood or rubber-backed aluminum are well known and widely used in the art.
An example of an adjustable height scaffold which is designed for use with an upright constructed of wood is U.S. Pat. No. 2,216,912 to Hoitsma. The Hoitsma patent discloses an angle bracket to which a jack is coupled, this type of bracket assembly being commonly referred to simply as a pump jack in the art. Another example of an adjustable height scaffold which is designed for use with an upright constructed of wood is U.S. Pat. No. 2,342,427 to Riblet. The Riblet '427 patent discloses a bracket assembly which is raised and lowered by a block and tackle, this type of bracket assembly being commonly referred to simply as a Painter's Pole in the art.
Examples of adjustable height scaffolds which are designed for use with an upright constructed of rubber backed aluminum include U.S. Pat. No. 4,597,471 to Anderson and U.S. Pat. No. 5,259,478 to Berish et al. It should be noted that the Anderson patent and the Berish patent both adapt aspects of the pump jack mechanism disclosed in the Hoitsma patent noted above. It should also be noted that aspects of the Painter's Pole mechanism disclosed in Riblet '427 for adjustable height scaffolds have been adapted for use on rubber-backed aluminum uprights.
U.S. Pat. No. 2,801,851 to Meek and U.S. Pat. No. 2,891,759 to Holmboe disclose related bracket assemblies. The bracket assemblies disclosed in Meek and Holmboe differ from the bracket assemblies noted above in that the bracket assemblies disclosed in Meek and Holmboe include an inner jaw which is formed from an extended surface of its associated angle bracket. It should be noted that the bracket assemblies disclosed in Meek and Holmboe were not intended for use in adjustable height scaffolds. Accordingly, it is not clear how reliably the lock of Meek and/or Holmboe would grip an upright if utilized for the height adjustment of a scaffold.
The load supporting lock of the pump jack is pivotally connected to a pair of linkage members which, in turn, are pivotally connected to the angle bracket of the adjustable height scaffold. In contrast, the load supporting lock of the Painter's Pole is directly connected by a pivot bar to the angle bracket.
As can be appreciated, the implementation of a linkage in the pump jack is significant in that the load placed on the angle bracket is not transferred directly to the lock. As a result, a spring is required to initiate contact of the jaws of the lock with the upright. To the contrary, in the Painter's Pole, the lock is directly coupled to the angle bracket so that a load placed on the angle bracket always exerts a turning moment on the lock, such a lock being commonly referred to as a load actuated lock in the art.
A load actuated lock, by definition, is pivotally coupled directly to the angle bracket. Furthermore, in the Painter's Pole, the lock is pivotally coupled to the angle bracket in such a manner so that the application of a turning moment onto the lock brings both jaws of the lock into contact with the upright when the width of the upright exceeds the desired design width, thereby activating the lock. Accordingly, a load actuated lock which functions in this manner is often referred to simply as a load activated lock.
It should be noted that, upon the placement of a load onto the angle bracket of a pump jack, crossbars at the upper and lower ends of the angle bracket exert horizontal forces onto the upright. By contrast, upon the placement of a load onto the angle bracket of a Painter's Pole, the horizontal force on the load actuated lock created by the offset load is directly applied onto the upright. As such, the horizontal forces exerted on an upright by the jaws of the Painter's Pole load actuated lock are substantially greater than the horizontal forces exerted on an upright by the jaws of the pump jack, spring activated lock. As a result, in order to obtain the same load supporting vertical force as the Painter's Pole, the jaws of the pump jack lock require a greater coefficient of friction with the surface of the upright.
Consequently, it is well known in the art for the outer jaw of commercial pump jacks to include a sharpened edge to increase the coefficient of friction between the outer jaw and the surface of an upright. In contrast, the jaws of the lock of a Painter's Pole comprise smooth, cylindrical surfaces which contact the surface of a wooden upright along the points of a cylinder. As a result of its sharpened edge, the jaw of a pump jack lock requires less horizontal force than the rounded edged jaws of Painter's Pole locks in order to provide the same vertical, frictional force on a wooden upright.
When used on metal uprights, such as aluminum uprights, pump jack locks and Painter's Pole locks typically retain the similar types of jaws. Specifically, when used on metal uprights, pump jack locks comprise a sharpened edged outer jaw and a rounded edged inner jaw whereas Painter's Pole locks comprise rounded edged inner and outer jaws. Both types of jaws are referred to as “edged” jaws because both contact a flat faced aluminum upright along a line.
It should be noted that the edged outer jaw of a pump jack lock and the edged jaws of a Painter's Pole lock introduces a significant drawback. Specifically, it has been found that the edged outer jaw of a pump jack lock as well as the edged jaws of the Painter's Pole lock tend to gall or mar a metal upright, which is highly undesirable.
Accordingly, in order to minimize the galling or marring of an aluminum upright by the edged jaws of a pump jack lock or the edged jaws of a Painter's Pole lock, aluminum uprights are backed with a strip of rubber.
However, it has been found that backing an aluminum upright with a strip of rubber introduces notable drawbacks. Specifically, backing an aluminum upright with a strip of rubber significantly increases both the weight and the cost of the upright without adding to the strength of the upright. In addition, backing an aluminum upright with a strip of rubber introduces as a key factor, in the level of safety of the lock on the upright, the reliability of the rubber backing.